1. Field of the Invention
This invention relates generally to devices for measuring the level of liquids in containers, and particularly to a float based measuring system for the accurate determination of changing levels of the liquid contents of a container.
2. Description of the Related Art
One of the best known techniques for determining the level of liquid in a container makes use of a float supported by the liquid, the level of the float being determined by means of a suitable transducer which converts vertical movement of the float into signals which can be displayed remotely or utilized for control purposes.
According to the nature of the liquid whose level is being monitored, which may be flammable, corrosive or have other undesirable properties, or produce vapours or residues with such properties, it may be necessary either to protect the transducer from contact with the liquid or vapour, or in the case of flammable or explosive vapours and electrical transducers, to isolate the vapours from the transducer.
Ultrasonic pulse-echo ranging systems have also been widely utilized for determining the level of liquids and solids in tanks, silos, channels and other containers. Examples of such systems are described in U.S. Pat. Nos. 4,596,144 (Panton et al), 4,831,565 (Woodward) and 4,890,266 (Woodward). A problem when using such equipment in sealed containers, containers containing volatile materials, or containers subject to substantial temperature variation, is that the speed of sound varies markedly both with temperature and according to the composition of the atmosphere between the ultrasonic transducer and the liquid level being measured. Furthermore, the high electrical voltages required to energize the transducer can provide problems in meeting safety requirements, especially where highly flammable vapours emitted from the contents of the tank are involved.
In order to address the above problems, various techniques have been developed for indirectly sensing the position of a float in a container. Such techniques generally utilize an annular float surrounding a vertically extending waveguide element. Wave energy is transmitted from one end of the element and reflected at the position of the float back to a receiver so that its transit time can be utilized to determine the position of the float indirectly. Examples of such arrangements are to be found in U.S. Pat. Nos. 3,372,592 (Gravert); 4,158,964 (McCrea et al); 4,305,283 (Redding); and 4,839,590 (Koski et al).
In U.S. Pat. No. 4,589,282, the annular float contains magnets, and surrounds a vertical tube containing a vertical array of Hall-effect sensors. Despite the "vernier" feature described in the patent, a very large number of sensors is required, and the electronic portion of the device is necessarily complex.
In U.S. Pat. No. 4,337,656 (Rapp), indirect level measurements are obtained by providing a float on the surface of the contents of the tank which surrounds a vertical tube inserted into the contents of the container but isolated from its contents. The float contains magnets supporting a follower which is located inside the tube and which is attracted by the magnets. Transducers located at each end of the tube act as transceivers for acoustic signals which are reflected by a target formed by the follower. By measuring the transit time of the signals the level of the target, and indirectly, the level of the float on the surface of the contents of the container, can be calculated.
However, it has been found that the target may not move freely inside the sealed container, especially where there are aberrations in the side wall of the tube, such as welded joints, and may therefore not provide an accurate reading of the level of contents in the container at all times.
It is therefore an object of the present invention to provide an indirect measurement system of the above general type, in which the target can move more freely throughout the length of the tube container, in order to yield more accurate level readings.